A material under tension or compression undergoes a change in both length and width. For example, for a tensile load, the ratio of the relative contraction strain (normal to the applied tensile force) to the relative extension strain (parallel to the applied force) is generally known as the Poisson's ratio. When a tensile force is applied to a material having a positive Poisson's ratio, the width of the material tends to decrease as the length of the material increases. Conversely, when a compressive force is applied to a material having a positive Poisson's ratio, the width of the material tends to increase as the length of the material decreases.
However, not all materials have a positive Poisson's ratio. Materials having a negative Poisson's ratio are commonly referred to as auxetic materials. For example, when a tensile force is applied to an auxetic material, the width of the material tends to increase as the length of the material increases. Conversely, when a compressive force is applied to an auxetic material, the width of the material tends to decrease as the length of the material decreases.
An auxetic material may exhibit a negative Poisson's ratio for only a portion of the total deformation of the material. For example, the Poisson's ratio may initially be negative; however, continued deformation of the auxetic material may result in the Poisson's ratio transitioning to a positive value. This may be the result of the internal structure of the auxetic material buckling. Accordingly, designers seeking to exploit the properties of an auxetic material are constrained, at least, by the transition point at which the Poisson's ratio becomes positive.